HMRC’s “tax year basis” concession for businesses and records.
Now we have everyone on SA transitioned to the tax year instead of their own choices, which becomes suddenly relevant where it never was before.
Our official personal and corporation tax year runs from 6 April to the following 5 April. However, HMRC accepts that for practical purposes, especially for bookkeeping and accounts preparation, we can treat the year ending 31 March as if it were the same as 5 April.
This is called the 31 March year-end concession [factual]: • Why: It avoids having to split income and expenses across a small stub period (1–5 April), since the difference is only five days. • Who it applies to: Self-employed people, partnerships, and landlords most commonly, but it also turns up in PAYE and other HMRC forms where “tax year” is relevant. • Effect: If records are to 31 March, HMRC will accept them as covering the full tax year to 5 April, with no adjustment required for those extra days. • Notification: There’s no special separate notification or election form needed — we simply prepare the accounts to 31 March and treat them as the tax year. HMRC’s online return and paper forms explicitly allow for this. If we were moving to or from a different accounting date, then we would have to notify them in the relevant self-assessment pages.
The exact HMRC reference is in their Self Assessment Manual at SAM121050 and in the Business Income Manual at BIM81045, which confirm that 31 March is deemed equivalent to 5 April for tax purposes.
What’s the 31 March Concession? • HMRC treats a business’s accounting period ending on 31 March (or any date between 31 March and 5 April) as if it ended on the tax year‐end, 5 April, for the purposes of basis-period calculations. This means we do not have to apportion profit for the extra days — it’s treated as if the accounts run through the full 5 April.  • The late accounting date rules under HMRC confirm that, for accounting dates between 31 March and 4 April inclusive, the profits from 1 April to 5 April are treated as nil for that tax year and instead attributed to the next tax year — unless we specifically elect otherwise. 
In practice: If our business prepared accounts to 31 March, in most years HMRC accepts that as covering profits to 5 April with no separate notification or special reporting needed — it’s built into the system.
Transition & Notification • There is no need for a separate notification to HMRC if you’re simply using a 31 March year-end — it’s accepted by default under the basis-period concession.  • However, if our accounting date is between 31 March and 4 April inclusive, the late accounting date rules apply automatically — unless you choose to elect out. That election must be made within one year of the normal Self Assessment filing deadline for that tax year. 
| Situation |
|---|
———————————————————– | | Accounts end on 31 March (or between 31 March–5 April) | Treated as ending on 5 April; profits 1–5 April attributed to the next tax year automatically | No | | Accounts end on 31 March–4 April | Profits 1–5 April treated as nil for current year, shifted to next year; optionally you can elect otherwise | Only if you want to override the default — yes (within 1 year) |
Situation What Happens Notification Required? Accounts end on 31 March (or between 31 March–5 April) Treated as ending on 5 April; profits 1–5 April attributed to the next tax year automatically No Accounts end on 31 March–4 April Profits 1–5 April treated as nil for current year, shifted to next year; optionally you can elect otherwise Only if you want to override the default — yes (within 1 year)
So HMRC allows a 31 March year-end to “stand in” for the 5 April tax year-end. No special filing or notification required, unless we’re deliberately wanting to disapply the late accounting date treatment.